Thermal emittance and lifetime of alkali-antimonide photocathodes grown on GaAs and molybdenum substrates evaluated in a −300  kV dc photogun

Abstract

Cs x K y Sb photocathodes grown on GaAs and molybdenum substrates were evaluated using a −300 kV dc high voltage photogun and diagnostic beam line. Photocathodes grown on GaAs substrates, with varying antimony layer thickness (estimated range from <20 nm to >1 um), yielded similar thermal emittance per rms laser spot size values (∼0.4 mm mrad=mm) but very different operating lifetime. Similar thermal emittance was obtained for a photocathode grown on a molybdenum substrate but with markedly improved lifetime. For this photocathode, no decay in quantum efficiency was measured at 4.5 mA average current and with peak current 0.55 A at the photocathode.

Photoemission from Bialkali Photocathodes through an Atomically Thin Protection Layer

“…Metals (e.g., Cu, Nb, Pb, and Mg) are robust photocathode materials and can operate for years under a vacuum condition of ∼10 –9 Torr. However, their relatively high work function (>4 eV) requires UV light for an excitation source, and the QEs are low (<0.1% at 4.66 eV). − Semiconductor photocathode materials, on the other hand, for example, alkali antimonides (Cs 3 Sb, Na 2 KSb, and K 2 CsSb), can have low work functions (<2 eV) and high QEs (>10% at 3 eV) and thus are widely used in high-sensitivity photomultipliers and high current photoinjectors. , One disadvantage however is that the alkali films are very reactive and can typically only maintain their high QE for less than a week even in ultrahigh vacuum (∼10 –11 Torr). − To overcome the rather inverse relationship between the lifetime and QE in order to achieve long lifetime and high QE simultaneously, we have employed a method of using an atomically thin protection barrier consisting of single- or few-layer graphene. − The choice of graphene as a protection barrier is directed by two of its important properties: gas impermeability and atomic thinness. − Bilayer graphene has been demonstrated to be impermeable to most gases (hydrogen, helium, nitrogen, oxygen, etc. ), and single-layer graphene is permeable only to hydrogen, and we have demonstrated previously that graphene and reduced graphene oxides present effective gas barriers for environmentally susceptible organic electronic material or metal photocathodes. , Since the degradation of alkali antimonides can be caused by the adsorption and reaction of residual gas molecules (H 2 O, CO 2 , and O 2 ), the nearly perfect impermeability of single and few-layer graphene makes it an attractive candidate for a protection layer against degradation.…”

Review of photocathodes for electron beam sources in particle accelerators

“…The thermal emittance was reported with 0.4 mm mrad mm À1 per rms laser spot size. 107 5.2.4 Lifetime. Wang et al investigated in the occurrence of multi-pacting and its influence on the quality of different antimonide cathodes operating in the SRF Gun.…”

Overview of the Semiconductor Photocathode Research in China

“…With the development of photocathode injectors, the intrinsic emittance of photocathode has become the main part of the final electron beam emittance and directly sets the minimum achievable emittance from the start. For an ultra-low emittance beam required application such as XFEL, it is critical to use a low intrinsic emittance photocathode or eliminate the intrinsic emittance of the photocathode with different methods [16][17][18][19][20][21][22].…”